Interleukin-7 (IL-7) signaling is essential for the development and peripheral maintenance of several blood cell types. Deficiencies in IL-7 or either component of its receptor result in major hematopoietic defects, causing T-cell-negative (T-), B-cell-negative (B-), natural killer cell-negative (NK-) severe combined immunodeficiency (SCID) in mice. In humans, mutations in the IL-7 receptor alpha (IL-7Ra) gene have been found to cause T-, B+, NK+ SCID, and more recently a single nucleotide polymorphism in the IL-7Ra gene has been associated with autoimmunity. In addition, IL-7 has been shown to be a critical mediator of peripheral T- cell homeostasis, survival, and function. Nevertheless, the molecular mechanism underlying the diverse functions of IL-7 remain poorly understood. We aim to uncover a new mechanism by which IL-7 signaling may regulate both T-cell development and peripheral function, namely through regulation of N-glycosylation. Insight in these areas may lead to new therapeutic avenues for both autoimmunity and congenital, acquired and induced lymphopenias. The roles of IL-7 are intricately involved with T-cell receptor (TCR) signaling, such that many of the effects of IL-7 correlate with alterations in TCR signal strength. N-glycosylation is a co-translational modification that affects nearly all cell-surface and secreted proteins. Cell surface glycoproteins interact with galectins in proportion to the number and branching of attached N-glycans, forming a molecular lattice that globally regulates the concentration and signaling of surface receptors. In T-cells, N-glycan branching opposes TCR clustering and signaling to suppress T-cell growth and autoimmunity. Our preliminary data demonstrate that IL-7 signaling leads to dramatic changes in N-glycan branching in T-cells. IL-7Ra mutant mice display a two to three fold increase in T-cell N-glycan branching, among the largest changes observed outside of genetic manipulation of Golgi enzymes. This hyper-glycosylation is predicted to markedly inhibit TCR signal strength and thus TCR signal mediated development, survival, and proliferation. We hypothesize that IL-7 regulates T-cell development and growth by altering N-glycan branching. To examine this hypothesis we propose to rescue hyper-glycosylation in IL-7Ra deficient mice by additionally knocking Mgat1 and Mgat5, two key N-glycan branching enzymes. This will allow us to directly asses the role of N-glycosylation in IL-7 mediated hematopoietic development, peripheral function, and homeostasis by comparing these processes in IL-7Ra-/- mice to mice with additional defect in glycosylation enzymes. These studies will provide insights about the interaction of IL-7 and TCR signaling, leading to a greater understanding of the development and maintenance of the lymphoid compartment. Furthermore, discerning the role of N-glycosylation in IL-7 signaling may provide new therapeutic avenues for both immunodeficiencies and autoimmune disease.